Is there sufficient contrast between text and background colors?
A key usability aid for the majority of web site users is to make sure that there is a high contrast between the color of text and the color of its background. This is particularly important for people with visual impairments that affect visual acuity or color perception.
When there is insufficient contrast between text and its background, some people will find it difficult or impossible to read.
Is information available without relying on color perception?
Color is an important asset in design of Web content, enhancing its aesthetic appeal, its usability, and its accessibility. Color is often used to distinguish classes of information, or indicate that action is required on a specific control.
However, some people have difficulty or are unable to perceive color. For example, people who are blind and use screen readers hear rather than see web content, so don’t have access to colors at all. Some people are unable to distinguish specific colors, for example red and green; while the ageing process can lead to reduced visual perception of certain colors.
In addition, some people may be using text-only, limited-color or monochrome displays and browsers, or accessing a black-and-white printed version of a page.
These user groups may be unable to access some information when it is presented only through color and not also available in another way.
Examples of information conveyed by color differences only might be in forms: "Errors are shown in red", or tables or charts: "Classes that are full are shown in red, classes with spaces available are shown in blue".
Examples of indications of an action include: using color to indicate that a link will open in a new window or that a database entry has been updated successfully. An example of prompting a response would be: using highlighting on form fields to indicate that a required field had been left blank.
All information should still be meaningful if color is removed.